The coating of wrought, cast and forged metal parts particularly ferrous parts, with sacrificial metals to provide atmospheric corrosion protection is a common practice in industry. Both the coating metal to be used and its thickness are dependent on the anticipated exposure conditions, pollution inhibitions, and cost restrictions. These coatings may be applied by various techniques known in the art including electroplating, mechanical plating, vacuum coating, or hot-dip galvanizing. Each method has advantages and disadvantages depending on the application, and thus the particular method chosen by one skilled in the art will depend on the particular applications and the desired results. Vacuum coating is expensive and is used for metals which are difficult to deposit. Electroplating and mechanical plating can be used to coat with several metals, while hot-dip galvanizing is limited to zinc coatings, all at a low cost. Mechanical plating and hot-dip galvanizing can apply thick coatings for greatly extended corrosion protection.
Many of the metal parts to be coated are ferrous parts made of high carbon steel which have their strength improved by various heat treating and/or work hardening processes. Such parts usually require cleaning in strong acids, commonly having dissociation constants of at least 10.sup.-3. In these processes, hydrogen is generated and absorbed by the parts, creating the possibility of hydrogen embrittlement. Mechanical plating, with its somewhat porous coating which permits rapid and spontaneous outgassing, virtually eliminates the possibility of hydrogen embrittlement, even with coatings as thick as 0.003 inches. No post plating heat treating for stress relief is required. A mechanical coating that is thin (as in electroplating) but providing the corrosion protection of thick hot-dip galvanizing would be advantageous since it would also avoid the problem of hydrogen embrittlement.
A practical limitation of the above coating systems has been the limitation of choices of metals that can be coated. Cadmium, which can be deposited by both electroplating and mechanical plating commercially is increasingly limited by pollution constraints. Zinc, which can be deposited by all three coating systems, is good in protecting against "red" rust when deposited in sufficient thickness, but in comparatively thin coatings, (e.g., 0.002 to 0.003 inches), zinc requires a post-plating chromate dip for extended corrosion resistance, another pollution hazard. Mechanical plating has had some success in co-depositing metals for additional coating utility, notably zinc/cadmium and tin/cadmium. Neither technique has eliminated the presence of cadmium.
It is common practice in mechanical plating to deposit a tin flash on the part to be coated by adding a, metal to a solution of a tin salt during the plating process such that the tin is galvanically reduced to tin metal. The common metals for this procedure are zinc, when zinc plating, or cadmium, when cadmium plating. This metal is called the driving metal. Golben has suggested in U.S. Pat. No. 3,400,012 that aluminum is suitable as a driving metal to produce the required tin flash. Coch has suggested in U.S. Pat. No. 4,654,230 that mechanical plating of aluminum alone can be improved by using pre-shaped aluminum particles. However, aluminum by itself proved a less than desirable coating in terms of corrosion resistance and physical properties.
Energy generation during mechanical plating processes is largely achieved by a combination of plating barrel design and speed of rotation. Barrels for mechanical plating are generally designed with six (or eight) sides to achieve a "bumping" or "throwing" motion. The energy generation achieved by plating barrel design is measured in surface feet/minute, and the surface feet/minute can be calculated for a given barrel as follows: Surface ft/min.=barrel major diameter (ft) x R.P.M.x3.1416 Surface speeds of 130-180 feet per minute suffice for virtually all parts when plating cadmium, tin or zinc.